That's according to a Harvard researcher measuring the CO2 output of several Google servers "competing against each other" to return search results super-quickly. Check out the Times of London article for a (really) detailed explanation.

UPDATE: As noted by commenters, Google has responded to the Times' article with a firm denial, claiming that, from their own math:

In terms of greenhouse gases, one Google search is equivalent to about 0.2 grams of CO2. The current EU standard for tailpipe emissions calls for 140 grams of CO2 per kilometer driven, but most cars don't reach that level yet. Thus, the average car driven for one kilometer (0.6 miles for those in the U.S.) produces as many greenhouse gases as a thousand Google searches.

So, if you regularly drink stove-boiled tea while searching online, you're now free to decide whose numbers seem truer, and if that results in true environmental impact.